In recent years, as large-capacity optical communication develops, introduction of a digital coherent system capable of expanding the capacity by phase interference progresses. The digital coherent system is a technique, in which a high-speed optical signal is converted by photoelectric conversion with a light receiving element, and is processed in a high-speed digital circuit for compensating an influence caused by dispersion or the like so as to implement long-distance and large-capacity transmission.
In a digital coherent system, a high-speed signal of 100 Gbps is handled, which requires high-speed response performance for a light receiving element. As one of the measures for attaining high speed performance for a light receiving element, it is a general practice to dispose a bypass capacitor in the vicinity of a photodiode (PD) to stabilize the impedance in a high frequency operation. As a related art, PTL 1 describes an example, in which an avalanche photodiode (APD) and a bypass capacitor are electrically connected to each other on a circuit substrate.
Further, as illustrated in FIG. 9, a digital coherent receiver is internally provided with, for instance, an optical fiber, a mixer, a PD, a lens, a capacitor, and a transimpedance amplifier (TIA). A mixer of an optical waveguide type is mainly used as the mixer. In this configuration, generally, the light exit position is at a height of about 0.5 to 1 mm depending on the thickness of a substrate of an optical waveguide. Light incident on the digital coherent receiver is reflected on a reflection mirror at 90 degrees, and is received by the PD on a substrate. PTL 2 describes an example, in which an related optical system is constituted by a lens substrate provided with a mirror surface, and an optical element.
On the other hand, as an optical module is miniaturized by a trend in recent years, there is a demand for miniaturization of a component to be used in a digital coherent module as well. In particular, it is essential to miniaturize an optical system for use in a digital coherent receiver. In view of the above, it is possible to miniaturize the optical system by omitting the aforementioned reflection mirror and by optically and directly connecting a lens and a PD. As a technique related to an optical system part, PTL 3 discloses a receptacle for receiving an optical connector plug. In the invention of PTL 3, laser light emitted from a semiconductor laser is collected on a lens, and the collected light enters a transparent bottom portion through the back surface of the lens. Laser light transmitted through the bottom portion is incident into an optical fiber from an inner bottom surface of a blind hole.